The present invention relates to a method and an apparatus for mounting electronic components, each having many leads with a narrow pitch, to a substrate such as a printed board or the like.
Various methods have been proposed heretofore to mount electronic components, each having a plurality of leads projected in parallel in two or four directions, to a substrate on the surface of which lands are formed correspondingly to the leads. One example of the methods will be described with reference to the related drawings hereinbelow. FIGS. 6A and 6B are a plan view and a longitudinal sectional view indicating the general mounting state of an electronic component to a substrate. FIGS. 7A and 7B are longitudinal sectional views of the substrate in a process of printing/applying a solder paste according to the conventional method, and FIGS. 8A and 8B are longitudinal sectional views of the substrate and an electronic component in a mounting and reflow process. In each figure, reference numerals 10-17 respectively represent the electronic component, the substrate, a land, a lead of the electronic component 10, a mask, the solder paste, a squeegee, and solder shortcircuiting the lands 12. In the solder paste printing/applying process first carried out for mounting the electronic component, as shown in FIG. 7A, the mask 14 is positioned and overlapped over the substrate 11, and the squeegee 16 kept in touch with the mask 14 with a suitable printing pressure is moved linearly along a printing direction, so that the solder paste 15 is filled in opening parts 14a of the mask 14. When the mask 14 is subsequently separated from the substrate 11, the solder paste 15 is printed and applied onto the substrate 11 via the mask 14 as shown in FIG. 7B. In the next mounting process, the electronic component 10 is held by suction by a mounting nozzle so that the leads 13 of the electronic component 10 are registered with the corresponding lands 12, as indicated in FIG. 8A. The leads 13 of the electronic component 10 are placed on the solder paste 15 printed/applied on the lands 12, whereby the leads 13 of the electronic component 10 are fixedly held by the adhesion force of the solder paste 15. As a result of this, the electronic component 10 is sent to the next process in the fixed state. In the final reflow process, the leads 13 of the electronic component 10 are heated by hot wind or a heat source, e.g., infrared heater or the like, so that the solder paste 15 printed/applied in the previous process is melted. The leads 13 of the electronic component 10 are thus solder-bonded to the lands 12.
The recent rapid trend of the electronic components such as IC package components and the like becoming more compact and of a thinner structure induces a narrower pitch of the leads 13 of the components, specifically, 0.5 mm to 0.4 mm and even not larger than 0.3 mm. Naturally, the lands 12 on the substrate 11 holding the above thin and compact electronic component 10 consequently are to be formed with a reduced pitch. However, although it is easy to form the lands of not larger than 0.3 mm pitch according to the current technique, there is a performance difference among the substrate manufacturers to form a solder resist between lands which is done generally with a pitch of 0.5 mm or 0.4 mm, in other words, it is not always possible to form the solder resist with such a narrow pitch as referred to above for every substrate.
In this situation, when the electronic component 10 with a narrow pitch of the leads is mounted with the use of the substrate 11 where the solder resist is not formed between the lands 12, the land 12 may be shifted from the opening part 14a when the substrate 11 is positioned and overlapped with the mask 14 as shown in FIG. 7A, and in consequence the solder paste 15 filled in the opening part 14a by the squeegee 16 leaks out from a gap X between the land 12 and the opening part 14a eventually to come in contact with an end part of the adjacent land 12 as is clearly shown in FIG. 7B. If the lead 13 of the electronic component 10 is placed on the corresponding land 12 in the state as shown in FIG. 8A and the reflow treatment is performed, the solder 17 might shortcircuit the lands 12 adjacent to each other as denoted by Y in FIG. 8B.